Gate structures ("word lines") for memory devices are currently fabricated using polycide processes. This process consists of the blanket deposition of a polysilicon layer 30 followed by a blanket deposition of a low resistivity layer 32, typically a metal silicide such a tungsten silicide. This layer can be formed either by direct deposition or metal deposition followed by a reaction anneal step. An overlying layer of silicon nitride 34 is then deposited. The layer structure is then patterned and etched to form narrow lines. A schematic of the gate structure (in cross section) made by this process is shown in FIG. 1.
The sheet resistance of these word line structures is an important consideration, and the current material in use, tungsten silicide (WSi.sub.2), has a resistivity of approximately 90 micro-ohm centimeter. This resistivity is unacceptably high for DRAM generations of greater than or equal to 256 MBit. For this reason, a lower resistivity material such a tungsten metal and Ti-silicide (TiSi.sub.2) could be used in this application. Subsequent thermal processing of the devices after gate formation involves high temperature anneals, typically greater than or equal to 800.degree. C., one of which is in an oxygen ambient atmosphere. This high temperature anneal in oxygen ambient atmosphere is called "smiling oxidation." All of the layers in the stack must be stable through these processes.
If tungsten, for example, is used as the gate material, the "smiling oxidation" step can cause a serious problem, since tungsten oxidizes easily at these temperatures and forms a volatile oxide. Additionally, a barrier must be interposed between the tungsten and the underlying polysilicon in the gate structure in order to prevent reaction between the two layers. Titanium nitride (TiN) can be used readily for this purpose, and will be exposed to the same thermal treatments as the rest of the stack. Like tungsten, Ti-nitride also oxidizes readily in the "smiling oxidation" ambient atmosphere. Other metals would have similar oxidation problems and barrier requirements. The basic problem, therefore, that is addressed by this invention is a method to yield thin, low resistance metal/polysilicon gate structures with a barrier layer between the metal and polysilicon that can withstand the "smiling oxidation" treatment.
Tungsten oxidation in the formation of gate structures has been an ongoing problem. One of the basic methods of reducing the oxidation of the tungsten in the gate formation is the selective oxidation of silicon rather than tungsten. Thermodynamically, for certain ratios of hydrogen and H.sub.2 O in the annealing ambient atmosphere, silicon dioxide (SiO.sub.2) will form, but tungsten oxide will not. This approach does not easily address the oxidation of the barrier layer, the ambient atmosphere for which tungsten is robust may not allow the barrier to survive through subsequent processing.
A second approach is to add a non-oxidizing side wall material, such as Si.sub.3 N.sub.4 to the tungsten gate stack before the "smiling oxidation" treatment, but after the patterning and etching step to form the gate stack. This is also a valid approach, except that the polysilicon will be coated with silicon nitride which would somehow need to be removed selectively in order to allow the polysilicon to be oxidized.
What is needed is a gate structure and associated method of making that has adequate sheet resistance and an encapsulated condition and barrier layer to resist subsequent processing.
It is with the foregoing problems in mind that the instant invention was developed.